Etching method

Abstract

The invention provides an etching method for removing a natural oxide film formed on the surface of a silicon nitride film. Comprises: etching a surface oxide layer of a silicon nitride film by supplying free radicals after supplying hydrogen fluoride gas to an object to be etched (steps S13, S14); and a cycle (steps S16 to S18) in which hydrogen fluoride gas is supplied to the object to be etched, the surface oxide layer of which has been etched, and then radicals are supplied is repeated a plurality of times, whereby the silicon nitride film is selectively etched with respect to the silicon oxide film, and the ratio of the etching amount of the silicon nitride film to the etching amount of the silicon oxide film is a selection ratio. A first selection ratio of a processing condition for etching the surface oxide layer is smaller than a second selection ratio of a processing condition for selectively etching the silicon nitride film.

Description

TECHNICAL FIELD

[0001] The present application relates to an etching method. BACKGROUND

[0002] One example of a method of selectively etching a silicon nitride film and a silicon oxide film in a wafer in which the silicon nitride film and the silicon oxide film are adjacent to each other includes a step of supplying a hydrogen fluoride gas to a processing space in which the wafer is housed, and a step of supplying radicals of an inert gas to the processing space. In the step of supplying the hydrogen fluoride gas and the step of supplying the radicals of the inert gas, the temperature of the wafer is maintained at a low temperature. In the etching method, first, the hydrogen fluoride is caused to be adsorbed to the surface of the silicon nitride film by supplying the hydrogen fluoride gas to the wafer. Next, the radicals of the inert gas are supplied to the wafer, whereby an energy equal to or higher than the activation energy of the etching reaction of the hydrogen fluoride and the silicon nitride is imparted to the wafer. As a result, etching of the silicon nitride film is performed (for example, refer to Patent Literature 1). PRIOR ART DOCUMENTS PATENT LITERATURE

[0003] Patent Literature 1: Japanese Patent Application Publication No. 2019-012759 SUMMARY PROBLEMS TO BE SOLVED BY THE INVENTION

[0004] However, sometimes a natural oxide film is formed on the surface of the silicon nitride film that is exposed to the outside. In this case, under etching conditions for selectively etching the silicon nitride film, etching of the natural oxide film does not easily proceed. Therefore, it is required to remove the natural oxide film formed on the surface of the silicon nitride film. SOLUTION TO THE PROBLEM

[0005] The etching method of the above aspect is a method of selectively etching a silicon nitride film in an etching object that includes the silicon nitride film and a silicon oxide film. The etching method includes: etching a surface oxide layer of the silicon nitride film by supplying a hydrogen fluoride gas to the etching object and then supplying radicals, and repeatedly performing a cycle including supplying the hydrogen fluoride gas to the etching object in which the surface oxide layer has been etched and then supplying radicals, thereby selectively etching the silicon nitride film with respect to the silicon oxide film. Furthermore, a ratio of an etching amount of the silicon nitride film with respect to an etching amount of the silicon oxide film is a selectivity ratio, and a first selectivity ratio possessed by a processing condition at the time of etching the surface oxide layer is smaller than a second selectivity ratio possessed by a processing condition at the time of selectively etching the silicon nitride film.

[0006] According to the etching method described above, when the surface oxide layer formed on the surface of the silicon nitride film is etched, the surface oxide layer is etched at a first selectivity ratio that is smaller than the second selectivity ratio. Therefore, the surface oxide layer can be removed without excessively etching the silicon oxide film before the selective etching of the silicon nitride film.

[0007] In the above etching method, etching the surface oxide layer may also include repeating the first cycle multiple times, wherein the first cycle includes: supplying the hydrogen fluoride gas to the etched object; and supplying the free radical to the etched object after the hydrogen fluoride gas has been supplied.

[0008] According to the etching method described above, the surface oxide layer is etched by repeating the first cycle multiple times. Therefore, the silicon oxide film is not over-etched, and the accuracy of etching the surface oxide layer throughout the silicon nitride film is improved.

[0009] In the above etching method, at least one of the free radicals used for etching the surface oxide layer and the free radicals used for selective etching of the silicon nitride film may be an oxygen-containing free radical. According to this etching method, hydrogen fluoride molecules adsorbed onto the etched object are activated by the oxygen-containing free radicals with relatively long free radical lifetimes.

[0010] In the above etching method, the cycle of supplying the free radical after supplying the hydrogen fluoride gas to the etched object whose surface oxide layer has been etched may be a second cycle. When etching the surface oxide layer, the amount of hydrogen fluoride molecules supplied to the etched object in one cycle is a first supply amount. When selectively etching the silicon nitride film, the amount of hydrogen fluoride molecules supplied to the etched object in one cycle of the second cycle is a second supply amount. The first supply amount is greater than the second supply amount.

[0011] According to the above etching method, since the first supply amount is greater than the second supply amount, the activated hydrogen fluoride molecules and the surface oxide layer readily react during the etching of the surface oxide layer. As a result, the first selectivity ratio can be made smaller than the second selectivity ratio.

[0012] In the above etching method, before etching the surface oxide layer, the etching object may be heated to a temperature between 80°C and 400°C, and the temperature of the etching object may be maintained within the range of between 80°C and 400°C during etching the surface oxide layer and selectively etching the silicon nitride film.

[0013] According to the above etching method, by heating the object to be etched to a temperature above 80°C, the adsorption amount of hydrogen fluoride molecules at different parts of the object is less likely to deviate. Furthermore, by heating the object to be etched to a temperature below 400°C, the temperature of the object is prevented from becoming excessively high, thus suppressing the adsorption of hydrogen fluoride molecules onto the object.

[0014] In the above etching method, the cycle of supplying the free radical after supplying the hydrogen fluoride gas to the etched object whose surface oxide layer has been etched may be a second cycle, and selectively etching the silicon nitride film includes: in one second cycle, etching the silicon nitride film to a thickness greater than 0 nm and less than 5 nm.

[0015] According to the above etching method, since the thickness of the silicon nitride film etched in one cycle is less than 5nm, the etching amount is not prone to deviation at various positions of the silicon nitride film. Attached Figure Description

[0016] Figure 1 This is a diagram showing the configuration of an etching apparatus. Figure 2 It means Figure 1 The diagram shows the configuration of the etching chamber in the etching apparatus. Figure 3 This is a flowchart used to illustrate the etching method. Figure 4 It is a timing diagram used to illustrate the driving of each supply unit in the etching apparatus. Figure 5 This is a process diagram used to illustrate a step in an etching method. Figure 6 This is a process diagram used to illustrate a step in an etching method. Figure 7 This is a process diagram used to illustrate a step in an etching method. Figure 8 This is a process diagram used to illustrate a step in an etching method. Figure 9 This is a process diagram used to illustrate a step in an etching method. Figure 10 This is a process diagram used to illustrate a step in an etching method. Figure 11 This is a process diagram used to illustrate a step in an etching method. Figure 12 This is a process diagram used to illustrate a step in an etching method. Figure 13 This is a process diagram used to illustrate a step in an etching method. Detailed Implementation

[0017] Reference Figures 1 to 13 This describes one implementation of the etching method. [Composition of Etching Apparatus 10] Reference Figure 1 Description of etching device 10.

[0018] like Figure 1 As shown, the etching apparatus 10 includes an etching chamber 11, a loading and locking chamber 12, and a gate valve 13. The etching apparatus 10 includes a free radical generating gas supply unit 21, a hydrogen fluoride gas supply unit 22 (HF gas supply unit), a plasma supply unit 23, and an inert gas supply unit 24. The etching apparatus 10 includes a control unit 10C.

[0019] Etching chamber 11 defines a processing space 11S (refer to) Figure 2 The processing space 11S houses a substrate S (refer to) that serves as an example of an etching object. Figure 2 The substrate S includes a silicon nitride film S1 and a silicon oxide film S2 (see reference). Figure 5 The etching chamber 11 etches the silicon nitride film S1 in the processing space 11S. The loading and locking chamber 12 moves the substrate S before etching from outside the etching apparatus 10 into the etching chamber 11. The loading and locking chamber 12 moves the etched substrate S from the etching chamber 11 out of the etching apparatus 10.

[0020] A gate valve 13 is disposed between the etching chamber 11 and the loading locking chamber 12. By opening the gate valve 13, the etching chamber 11 is connected to the loading locking chamber 12. By closing the gate valve 13, the etching chamber 11 is disconnected from the loading locking chamber 12.

[0021] The loading and locking chamber 12 is connected to the cooling gas supply unit 12A. The cooling gas supply unit 12A supplies cooling gas to the loading and locking chamber 12. The cooling gas is an inert gas used to cool the etched substrate S.

[0022] The etching chamber 11 includes a heating section 11A and an exhaust section 11B. The heating section 11A heats the etching chamber 11, thereby heating the substrate S within the processing space 11S. The exhaust section 11B depressurizes the etching chamber 11 to a predetermined pressure.

[0023] The etching chamber 11 is connected to the HF gas supply unit 22 and the plasma supply unit 23. The HF gas supply unit 22 supplies HF gas to the processing space 11S. The HF gas supply unit 22 is configured to supply hydrogen fluoride gas (HF gas) to the processing space 11S at a specified flow rate. The HF gas supply unit 22 is, for example, a mass flow controller.

[0024] The plasma supply unit 23 supplies plasma to the processing space 11S, thereby supplying the processing space 11S with the free radicals 33 contained in the plasma (see reference). Figure 8The plasma supply unit 23 includes a discharge tube 23A, a waveguide 23B, and a microwave irradiation unit 23C. The microwave irradiation unit 23C irradiates microwaves onto the discharge tube 23A through the waveguide 23B. The discharge tube 23A is connected to the free radical generating gas supply unit 21. The inner surface of the discharge tube 23A is composed of inorganic oxides. The inorganic oxides constituting the inner surface of the discharge tube 23A can be either silicon oxide or aluminum oxide. The discharge tube 23A can be, for example, a quartz tube.

[0025] The free radical generating gas supply unit 21 supplies free radical generating gas 32, which generates free radicals 33, to the discharge tube 23A. The free radical generating gas supply unit 21 supplies free radical generating gas 32 to the discharge tube 23A at a predetermined flow rate. The free radical generating gas supply unit 21 is, for example, a mass flow controller.

[0026] The free radical-generated gas 32 can be either a gas containing oxygen atoms or a noble gas. The noble gas can be either argon (Ar) or helium (He). The oxygen-containing gas can also be a mixture of oxygen, oxygen, and hydrogen, or nitrogen oxides (N...). x O y The nitrogen oxide gas may also be at least one of the following groups of gases: nitric oxide (NO), nitrogen dioxide (NO2), nitrous oxide (N2O), nitrous trioxide (N2O3), and nitrous pentoxide (N2O5).

[0027] The plasma supply unit 23 generates plasma in the discharge tube 23A by irradiating the free radical generating gas 32 with microwaves. The plasma contains hydrogen fluoride molecules (HF molecules 31, see reference). Figure 6 Activated free radical 33. The free radical 33 that activates HF molecule 31 can be an oxygen-containing free radical, an oxygen free radical, or a rare gas free radical.

[0028] Inert gas supply unit 24 supplies inert gas 35 to the processing space 11S. Inert gas supply unit 24 is configured to supply inert gas 35 to the processing space 11S at a specified flow rate. Inert gas supply unit 24 is, for example, a mass flow controller. Inert gas 35 can be either nitrogen (N2) or argon (Ar). Inert gas supply unit 24 can supply inert gas 35 to the processing space 11S using the same piping as HF gas supply unit 22, or it can use a separate piping.

[0029] The control unit 10C includes a storage unit 10CM. The storage unit 10CM stores process conditions for etching the silicon nitride film S1. The process conditions include the pressure of the etching chamber 11, the temperature of the substrate S, the flow rates of various gases, and the output of the microwave irradiation unit 23C. The control unit 10C controls the driving of the heating unit 11A, the exhaust unit 11B, the free radical generation gas supply unit 21, the HF gas supply unit 22, the plasma supply unit 23, and the inert gas supply unit 24 to ensure that the etching conditions are consistent with the process conditions.

[0030] While heating the heating unit 11A to maintain the substrate S at a predetermined temperature between a first temperature and a second temperature, the control unit 10C supplies HF gas to the HF gas supply unit 22 and then supplies free radicals 33 to the plasma supply unit 23.

[0031] The control unit 10C includes electronic circuits such as a CPU and MPU. The control unit 10C also includes storage devices such as SSDs and HDDs. Furthermore, the control unit 10C includes storage devices such as ROMs, RAMs, and register memories. The control unit 10C may also include integrated circuits such as ASICs and FPGAs. All processing performed by the control unit 10C can be executed either by the software it possesses, or by a combination of integrated circuits and software.

[0032] [Composition of etching chamber 11] like Figure 2 As shown, the etching chamber 11 houses a support portion 10A. The support portion 10A can support multiple substrates S. The multiple substrates S supported on the support portion 10A are stacked with gaps between adjacent substrates S. As described above, each substrate S has a silicon nitride film S1 and a silicon oxide film S2. One example of the substrate S has a circular plate shape.

[0033] One example of substrate S includes multiple silicon nitride films S1 and multiple silicon oxide films S2 (see reference). Figure 5 In a substrate S, silicon nitride films S1 and silicon oxide films S2 are alternately stacked. The substrate S has holes SA that penetrate multiple silicon nitride films S1 and multiple silicon oxide films S2. The sides defining the holes SA include end faces of each silicon nitride film S1 and each silicon oxide film S2. The end faces of each silicon nitride film S1 are oxidized. That is, a surface oxide layer SA1 of the silicon nitride film S1 is located on the end face of each silicon nitride film S1. The surface oxide layer SA1 of the silicon nitride film S1 is composed of at least one of silicon oxide and silicon oxynitride. The surface oxide layer SA1 of the silicon nitride film S1 can be composed of only silicon oxide, or of silicon oxynitride, or of both silicon oxide and silicon oxynitride.

[0034] The etching chamber 11 is equipped with a nozzle 11D. The nozzle 11D is connected to a discharge tube 23A. There can be one or more discharge tubes 23A connected to the nozzle 11D. Furthermore, Figure 2 An example is shown where two discharge tubes 23A are connected to a nozzle 11D. The nozzle 11D has multiple supply ports. The supply ports of the nozzle 11D are arranged along the direction in which the substrates S are stacked. The supply ports of the nozzle 11D supply plasma from the discharge tubes 23A toward the substrates S.

[0035] The etching chamber 11 includes a rotating part 11E. The rotating part 11E causes the support part 10A to rotate in the circumferential direction of the substrate S. The rotating part 11E also causes the plasma supplied from the nozzle 11D toward the substrate S and the HF gas supplied from the HF gas supply part 22 toward the substrate S to be dispersed in the circumferential direction of the substrate S.

[0036] The etching chamber 11 includes a temperature measuring unit 11F. The temperature measuring unit 11F measures the internal temperature of the etching chamber 11 as the temperature of the substrate S. The temperature measuring unit 11F is connected to the control unit 10C. The temperature measurement result of the temperature measuring unit 11F is input to the control unit 10C. The control unit 10C controls the driving of the heating unit 11A based on the measurement result of the temperature measuring unit 11F.

[0037] [Etching Method] Reference Figure 3 and Figure 4 Explain the etching method. The etching method is a method for selectively etching the silicon nitride film S1 in an etchable object comprising a silicon nitride film S1 and a silicon oxide film S2. The etching method includes etching a surface oxide layer SA1 formed on the silicon nitride film S1. The etching method includes selectively etching the silicon nitride film S1 relative to the silicon oxide film S2 in the etchable object after the surface oxide layer SA1 formed on the silicon nitride film S1 has been etched. The ratio of the amount of silicon nitride film S1 etched to the amount of silicon oxide film S2 etched, i.e., the ratio of the amount of silicon nitride film S1 etched relative to the amount of silicon oxide film S2 etched, is the selection ratio. The selection ratio of the processing conditions when etching the surface oxide layer SA1 is a first selection ratio. The selection ratio of the processing conditions when selectively etching the silicon nitride film S1 is a second selection ratio. The first selection ratio is less than the second selection ratio.

[0038] The etching of the surface oxide layer SA1 includes: supplying HF gas to the etched object; and supplying free radicals 33 generated by free radical generating gas 32 to the etched object after the HF gas has been supplied.

[0039] Selective etching of the silicon nitride film S1 is performed by repeating the second cycle multiple times. This second cycle includes: supplying HF gas to the object to be etched; and supplying free radicals 33 generated by free radical generating gas 32 to the object to be etched after the HF gas has been supplied. The free radical generating gas 32 used for selective etching of the silicon nitride film S1 may be the same as or different from the free radical generating gas 32 used for etching the surface oxide layer SA1.

[0040] According to the above etching method, when etching the surface oxide layer SA1 formed on the surface of the silicon nitride film S1, the surface oxide layer SA1 is etched at a first selectivity ratio that is less than the second selectivity ratio. Therefore, the surface oxide layer SA1 can be removed before the selective etching of the silicon nitride film S1.

[0041] Figure 3 This is a flowchart illustrating the etching method. Furthermore, the processing described below is performed by the control unit 10C executing the etching program, which includes the process conditions stored in the control unit 10C.

[0042] like Figure 3 As shown, the etching method includes a substrate setting process (step S11) and a heating process (step S12). In the substrate setting process, multiple substrates S are set on the support portion 10A. At this time, each substrate S is set in the support portion 10A disposed in the loading locking chamber 12. Next, the support portion 10A on which multiple substrates S are set is moved from the loading locking chamber 12 to the etching chamber 11. Then, after the gate valve 13 is closed, the exhaust portion 11B depressurizes the pressure in the etching chamber 11 to a predetermined pressure.

[0043] In the heating process, the heating unit 11A heats the substrate S. The heating unit 11A may also heat the substrate S to a temperature of 80°C to 400°C before etching the surface oxide layer SA1 of the silicon nitride film S1. The temperature of the substrate S is maintained within the range of 80°C to 400°C during the etching of the surface oxide layer SA1 and the selective etching of the silicon nitride film S1.

[0044] By heating the substrate S to a temperature of 80°C or higher, the adsorption amount of HF molecules 31 at various locations on the substrate S is less likely to deviate. In particular, when the substrate S has pores SA penetrating multiple silicon oxide films S2 and multiple silicon nitride films S1, the preferential adsorption of HF molecules 31 at the openings of the pores SA can be suppressed. Therefore, the adsorption amount of HF molecules 31 adsorbed onto the sides of the defined pores SA in the depth direction is less likely to deviate. Furthermore, by heating the substrate S to a temperature below 400°C, the temperature of the substrate S is prevented from becoming excessively high, thus suppressing the adsorption of HF molecules 31 onto the substrate S.

[0045] Next, the surface oxide layer SA1 formed on the surface of the silicon nitride film S1 is etched. The etching method includes: an adsorption process of HF molecules 31 (step S13); a supply process of free radicals 33 (step S14); and a process of determining whether the first cycle including the adsorption process and the supply process is repeated a predetermined M times (2≦M) (step S15).

[0046] In the adsorption process of HF molecules 31, the HF gas supply unit 22 supplies HF gas to the etching chamber 11. In the supply process of free radicals 33, the free radical generating gas supply unit 21 supplies free radical generating gas 32 to the discharge tube 23A. Next, the microwave irradiation unit 23C irradiates microwaves onto the discharge tube 23A, thereby generating plasma containing free radicals 33 within the discharge tube 23A. The plasma supply unit 23 supplies the plasma containing free radicals 33 to the etching chamber 11. The free radicals 33 may, for example, be oxygen free radicals (O free radicals).

[0047] The first cycle, which includes the adsorption process of HF molecules 31 and the supply process of free radicals 33, is repeated M times. The number of repetitions of the first cycle can also be preset. Thus, in the above etching method, the surface oxide layer SA1 is etched by repeating the first cycle multiple times. The first cycle includes supplying HF gas to the substrate S and supplying free radicals 33 to the substrate S after the HF gas has been supplied. Because the surface oxide layer SA1 is etched by repeating the first cycle multiple times, the accuracy of etching the surface oxide layer SA1 is improved.

[0048] Next, the substrate S after the surface oxide layer SA1 is etched is etched with a silicon nitride film S1. The etching method includes: an adsorption process of HF molecules 31 (step S16); a supply process of free radicals 33 (step S17); and a process of determining whether the second cycle including the adsorption process and the supply process is repeated a predetermined N times (2≦N) (step S18).

[0049] In the adsorption process of HF molecules 31, the HF gas supply unit 22 supplies HF gas to the etching chamber 11. In the supply process of free radicals 33, the free radical generating gas supply unit 21 supplies free radical generating gas 32 to the discharge tube 23A. Next, microwaves are irradiated onto the discharge tube 23A by the microwave irradiation unit 23C, thereby generating plasma containing free radicals 33 within the discharge tube 23A. The plasma supply unit 23 supplies the plasma containing free radicals 33 to the etching chamber 11. The free radicals 33 may be, for example, O free radicals.

[0050] The second cycle, which includes the adsorption process of HF molecules 31 and the supply process of free radicals 33, is repeated N times. The number of times the second cycle is repeated can also be preset. When selectively etching the silicon nitride film S1, a thickness greater than 0 nm and less than 5 nm can be etched into the silicon nitride film S1 in one second cycle. Because the thickness of the silicon nitride film S1 etched in one second cycle is less than 5 nm, the etching amount is less likely to deviate at each location of the silicon nitride film S1. In particular, when the substrate S has holes SA penetrating multiple silicon oxide films S2 and multiple silicon nitride films S1, the etching amount of the silicon nitride film S1 is less likely to deviate in the depth direction of the holes SA.

[0051] The etching method also includes a substrate recovery step (step S19). In the substrate recovery step, the pressure inside the etching chamber 11 is increased to be equal to the pressure inside the loading and locking chamber 12. Then, after opening the gate valve 13, the support 10A moves from the etching chamber 11 to the loading and locking chamber 12. Then, after closing the gate valve 13, the cooling gas supply unit 12A supplies cooling gas into the loading and locking chamber 12. And, after the temperature of the substrate S drops below a predetermined temperature, the cooling gas supply unit 12A stops supplying cooling gas. Then, the etched substrate S is recovered from the loading and locking chamber 12.

[0052] In the etching method, when etching the surface oxide layer SA1, the amount of HF molecules 31 supplied to the substrate S in one cycle is the first supply amount. That is, the amount of HF molecules 31 supplied to the substrate S in one first cycle is the first supply amount. When selectively etching the silicon nitride film S1, the amount of HF molecules 31 supplied to the etched target in one second cycle is the second supply amount. The first supply amount is greater than the second supply amount.

[0053] By making the first supply amount greater than the second supply amount, the activated HF molecules 31 and the silicon oxide film S2 react more readily. As a result, the first selectivity ratio can be made less than the second selectivity ratio.

[0054] Figure 4 An example is shown in which the heating unit 11A, the free radical generating gas supply unit 21, the HF gas supply unit 22, the microwave irradiation unit 23C, and the inert gas supply unit 24 are driven by the control unit 10C. Furthermore, Figure 4 This indicates the method of driving each part in the heating process (step S12) and in one first cycle.

[0055] In addition, Figure 4In this configuration, the state of stopping heating of heating unit 11A is indicated as "OFF," and the state of heating unit 11A is indicated as "ON." Similarly, the state of stopping microwave irradiation of microwave irradiation unit 23C is indicated as "OFF," and the state of microwave irradiation of microwave irradiation unit 23C is indicated as "ON." Furthermore, the state of stopping gas supply from each gas supply unit 21, 22, and 24 is indicated as "OFF," and the state of supplying gas from each gas supply unit 21, 22, and 24 is indicated as "ON."

[0056] like Figure 4 As shown, when etching the substrate S in the etching chamber 11, firstly, at time t0, the substrate S is placed in the etching chamber 11. Next, at time t1, the control unit 10C starts heating the heating unit 11A. As a result, the temperature T of the substrate S begins to rise, and at time t2, the temperature T of the substrate S reaches a predetermined temperature within the range of 80°C to 400°C.

[0057] Then, at time t3, the control unit 10C causes the HF gas supply unit 22 to start supplying HF gas. Next, at time t4, the control unit 10C stops the supply of HF gas to the HF gas supply unit 22 and starts the supply of free radical generating gas 32 to the free radical generating gas supply unit 21. Then, at time t5, the control unit 10C starts microwave irradiation to the microwave irradiation unit 23C.

[0058] Next, at time t6, the control unit 10C stops the supply of free radical generating gas 32 to the free radical generating gas supply unit 21, stops the microwave irradiation unit 23C from irradiating with microwaves, and starts the supply of inert gas 35 to the inert gas supply unit 24. Then, at time t7, the control unit 10C stops the supply of inert gas 35 to the inert gas supply unit 24.

[0059] Thus, in the process performed by the control unit 10C in the etching chamber 11, the heating of the substrate S begins at time t1, and the heating of the substrate S continues until the etching of the substrate S is completed. Furthermore, in the process performed by the control unit 10C in the etching chamber 11, the process from time t3 to time t4 involves supplying HF gas and causing it to adsorb onto the substrate S. Furthermore, the process from time t4 to time t6 involves supplying free radical generating gas 32. Furthermore, in the process performed by the control unit 10C in the etching chamber 11, the process from time t5 to time t6 involves supplying free radicals 33. Furthermore, the process from time t6 to time t7 involves supplying inert gas 35.

[0060] That is, the processing from time t3 to time t7 constitutes one first cycle. The control unit 10C causes the etching chamber 11 to perform multiple first cycles until the etching amount of the surface oxide layer SA1 in the substrate S reaches a predetermined amount. When repeating the first cycle, time t7 in the (m-1)th first cycle and time t3 in the mth first cycle can be simultaneous. Alternatively, time t3 in the mth first cycle can begin after a predetermined period has elapsed from time t7 in the (m-1)th first cycle.

[0061] Furthermore, in a second cycle, the same processing as in the first cycle from time t3 to time t7 is performed. However, the length from time t3 to time t4 in the first cycle is longer than the length from time t3 to time t4 in the second cycle. That is, the period for supplying HF gas in the first cycle is longer than the period for supplying HF gas in the second cycle. Consequently, the first supply amount in the first cycle is greater than the second supply amount in the second cycle. Furthermore, the final time t7 in the first cycle and the initial time t3 in the second cycle can occur simultaneously. Alternatively, the initial time t3 in the second cycle can begin after a predetermined period has elapsed from the final time t7 in the first cycle.

[0062] [effect] Reference Figures 5 to 13 Explain the function of the etching method. like Figure 5 As shown, the substrate S has a hole SA extending along the thickness direction. The hole SA penetrates two or more layers of silicon nitride film S1 and two or more layers of silicon oxide film S2. The substrate S includes a support substrate S3. A multilayer film including silicon nitride film S1 and silicon oxide film S2 is formed on the support substrate S3. Furthermore, in Figures 5 to 13 For ease of illustration, only one hole SA is shown, but the substrate S has multiple holes SA. The substrate S can also be, for example, a substrate for three-dimensional (3D) NAND devices.

[0063] like Figure 6 As shown, in the etching method, firstly, the etching apparatus 10 supplies HF gas to the substrate S. Consequently, the HF molecules 31 supplied to the substrate S are also supplied to the inner surface of the pores SA, including the surface oxide layer SA1. The HF molecules 31 have high adsorption capacity relative to the substrate S. Therefore, the amount of HF molecules 31 adsorbed near the opening of the pores SA is more likely to be greater than the amount adsorbed at the bottom of the pores SA. At this point, because the substrate S is heated, the HF molecules 31 supplied to the substrate S are less likely to be consumed (i.e., adsorbed) near the opening of the pores SA, and as a result, the HF molecules 31 are also more easily introduced to the bottom of the pores SA.

[0064] In the process of supplying HF gas to the substrate S, the pressure within the processing space 11S where the HF gas is supplied can be 500 Pa or higher. When the pressure within the processing space 11S is 500 Pa or higher, HF molecules 31 are easily adsorbed onto the substrate S.

[0065] like Figure 7 As shown, after supplying HF gas, the etching apparatus 10 switches the type of gas supplied to the substrate S from HF gas to radical generating gas 32. At this time, when the radical generating gas 32 is an oxygen-containing gas, its affinity for the silicon oxide film S2 is higher than that for HF molecules 31. Therefore, HF molecules 31 adsorbed onto the silicon oxide film S2 with the defined pores SA are replaced by the radical generating gas 32 with a higher probability than the thin oxide layer SA1 formed on the silicon nitride film S1. Furthermore, because the adsorption capacity of the radical generating gas 32 is low, it is less likely for it to remain on the silicon oxide film S2.

[0066] During the supply of HF gas to substrate S and the supply of free radicals 33 to substrate S, if free radical generating gas 32 is supplied to substrate S, at least a portion of the unwanted HF molecules 31 located on substrate S are removed from substrate S by the flow of free radical generating gas 32.

[0067] like Figure 8 As shown, after supplying the radical generating gas 32, the etching apparatus 10 supplies the substrate S with radicals 33 generated by the radical generating gas 32. Thereby, HF molecules 31 adsorbed onto the silicon nitride film S1 with defined holes SA and the radicals 33 generate an etchant 34 for etching the surface oxide layer SA1. Thus, by performing a surface reaction between HF molecules 31 and radicals 33 on the substrate S, the surface oxide layer SA1 is etched. The etching of the surface oxide layer SA1 proceeds in a direction orthogonal to the depth direction of the holes SA.

[0068] When etching the surface oxide layer SA1, the surface oxide layer SA1 is etched at a first selectivity ratio that is less than the second selectivity ratio. However, as described above, the HF molecules 31 adsorbed onto the silicon oxide film S2 are replaced by the free radical generating gas 32 with a higher probability than the thin oxide layer SA1 formed on the surface of the silicon nitride film S1. Therefore, before the selective etching of the silicon nitride film S1, the surface oxide layer SA1 can be preferentially removed compared to the silicon oxide film S2.

[0069] When the radical generating gas 32 is an oxygen-containing gas, the oxygen-containing radicals 33 are particularly difficult to deactivate, so the radicals 33 are easily supplied to the interior of the pore SA. Therefore, the surface oxide layer SA1 can be etched even inside the pore SA and near the bottom of the pore SA. Furthermore, when the radical 33 is generated from a gas containing oxygen atoms, the surface of the silicon oxide film S2, which has not adsorbed HF molecules 31, is repaired by the radical 33. Therefore, the surface oxide layer SA1 can be etched more preferentially than the silicon oxide film S2.

[0070] like Figure 9 As shown, after supplying free radicals 33 to the substrate S, the etching apparatus 10 switches the type of gas supplied to the substrate S from free radicals 33 to inert gas 35. As described above, inert gas 35 can be, for example, nitrogen. By supplying inert gas 35 to the substrate S, inert gas 35 is supplied into the holes SA, thereby replacing the HF molecules 31, free radicals 33, and etchant 34 remaining in the holes SA with inert gas 35. Furthermore, when the etching of the surface oxide layer SA1 is repeated in multiple cycles, the temperature of the substrate S is maintained at a predetermined value from the first cycle to the end of the last cycle.

[0071] As described above, in the etching method, a first cycle includes supplying HF gas to the substrate S and supplying free radicals 33 to the substrate S. The etching method may include repeating multiple first cycles. The first cycle includes supplying an inert gas 35 to the substrate S after supplying free radicals 33 to the substrate S. Thus, the free radicals 33 supplied to the etched object in the m-th first cycle are less likely to be present near the substrate S at the beginning of the (m+1)-th first cycle. Therefore, in the (m+1)-th first cycle, etching can be suppressed at locations other than the surface oxide layer SA1.

[0072] like Figure 10 As shown, after etching the surface oxide layer SA1, the etching apparatus 10 supplies HF gas to the substrate S again. As a result, the HF molecules 31 already supplied to the substrate S are supplied to the inner surface of the hole SA, which is formed by the end faces of the silicon nitride film S1 (from which the surface oxide layer SA1 has been etched) and the silicon oxide film S2. At this time, the substrate S is heated, causing the HF molecules 31 supplied to the substrate S to be consumed near the opening of the hole SA. Consequently, HF molecules 31 are also easily introduced into the bottom of the hole SA. The pressure within the processing space 11S where the HF gas is supplied during the process of supplying HF gas to the substrate S can be higher than the pressure within the processing space 11S where the HF gas is supplied during the etching of the surface oxide layer SA1.

[0073] like Figure 11As shown, after supplying HF gas, the etching apparatus 10 switches the type of gas supplied to the substrate S from HF gas to radical generating gas 32. At this time, when the radical generating gas 32 is an oxygen-containing gas, its affinity for the silicon oxide film S2 is higher than that for HF molecules 31. Therefore, the HF molecules 31 adsorbed onto the silicon oxide film S2 in the defined pores SA are replaced by the radical generating gas 32 with a higher probability than those on the silicon nitride film S1.

[0074] like Figure 12 As shown, after supplying the radical generating gas 32, the etching apparatus 10 supplies the free radicals 33 generated by the radical generating gas 32 to the substrate S. Thereby, HF molecules 31 adsorbed onto the silicon nitride film S1 and the free radicals 33 generate an etchant 34 for etching the silicon nitride film S1. Thus, the silicon nitride film S1 is etched by a surface reaction between HF molecules 31 and free radicals 33 on the substrate S. The etching of the surface oxide layer silicon nitride film S1 is performed along a direction orthogonal to the depth direction of the holes SA.

[0075] When etching the silicon nitride film S1, the silicon nitride film S1 is etched with a second selectivity ratio greater than the first selectivity ratio. At this time, HF molecules 31 adsorbed on the silicon oxide film S2 are replaced by the free radical generating gas 32 with a higher probability than those on the silicon nitride film S1. Therefore, selective etching of the silicon nitride film S1 is performed. In addition, when the free radical 33 is generated by a gas containing oxygen atoms, the surface of the silicon oxide film S2 that has not adsorbed HF molecules 31 is repaired by the free radical 33. As a result, the silicon nitride film S1 can be etched more preferentially than the silicon oxide film S2.

[0076] like Figure 13 As shown, after supplying free radicals 33 to the substrate S, the etching apparatus 10 switches the type of gas supplied to the substrate S from free radicals 33 to inert gas 35. As described above, inert gas 35 can be, for example, nitrogen. By supplying inert gas 35 to the substrate S, inert gas 35 is supplied into the holes SA, thereby replacing the HF molecules 31, free radicals 33, and etchant 34 remaining in the holes SA with inert gas 35. Furthermore, from the first second cycle to the end of the final second cycle, the temperature of the substrate S is maintained at a predetermined value. One second cycle includes supplying HF gas to the substrate S and supplying free radicals 33 to the substrate S. The second cycle includes supplying inert gas 35 to the substrate S after supplying free radicals 33 to the substrate S. As a result, the free radicals 33 supplied to the etching target in the nth second cycle are less likely to exist near the substrate S at the beginning of the (n+1)th second cycle. Therefore, in the (n+1)th second cycle, etching can be suppressed at locations other than the silicon nitride film S1.

[0077] As described above, according to one embodiment of the etching method, the following effects can be obtained. (1) When etching the surface oxide layer SA1 of the silicon nitride film S1, the surface oxide layer SA1 is etched at a first selection ratio that is less than the second selection ratio. Therefore, the surface oxide layer SA1 can be removed without over-etching the silicon oxide film S2 before the selective etching of the silicon nitride film S1.

[0078] (2) The surface oxide layer SA1 is etched by repeating the first cycle multiple times. Therefore, the silicon oxide film S2 is not over-etched, and the accuracy of etching the surface oxide layer SA1 throughout the silicon nitride film S1 is improved.

[0079] (3) When the free radical 33 is an oxygen-containing free radical, the HF molecule 31 adsorbed onto the etched object is activated due to the oxygen-containing free radical with a relatively long free radical lifetime.

[0080] (4) Since the first supply is greater than the second supply, the activated HF molecules 31 and the surface oxide layer SA1 are more likely to react during the etching of the surface oxide layer SA1. (5) By heating the substrate S to a temperature above 80°C, the adsorption amount of HF molecules 31 at various locations on the substrate S is less likely to deviate. In addition, by heating the substrate S to a temperature below 400°C, the temperature of the substrate S will not become too high, thus suppressing the adsorption of HF molecules 31 onto the substrate S.

[0081] (6) When the thickness of the silicon nitride film S1 etched in one first cycle is less than 5 nm, the etching amount is less likely to deviate at each position of the surface oxide layer SA1.

[0082] Furthermore, the above-described implementation method can be modified as follows. The etching method can also be configured such that, in the etching of the surface oxide layer SA1, the temperature of the substrate S is reduced compared to the selective etching of the silicon nitride film S1, thereby making the first selectivity ratio smaller than the second selectivity ratio.

[0083] The etching method can also be configured such that, during the etching of the surface oxide layer SA1, the pressure of the etching chamber 11 is increased compared to the selective etching of the silicon nitride film S1, thereby making the first selectivity ratio smaller than the second selectivity ratio.

[0084] The etching method can also be configured such that, in the etching of the surface oxide layer SA1, the flux of HF molecules 31 is increased compared with the selective etching of the silicon nitride film S1, thereby making the first selectivity ratio smaller than the second selectivity ratio.

[0085] The etching method can also be configured such that the free radical generating gas 32 in the etching of the surface oxide layer SA1 and the free radical generating gas 32 in the selective etching of the silicon nitride film S1 are different from each other, so that the first selectivity ratio is less than the second selectivity ratio.

[0086] The etching method can also make the first selectivity less than the second selectivity by varying at least one of the following: the supply period of HF molecules 31, the temperature of the substrate S, the pressure of the etching chamber 11, the flow rate of HF molecules 31, and the type of free radical generating gas 32. Explanation of reference numerals in the attached figures

[0087] S: Substrate S1: Silicon nitride film S2: Silicon oxide film SA1: Surface oxide layer T: Temperature 10: Etching apparatus 10C: Control Unit 11: Etching Chamber 11S: Processing Space 21: Free Radical Generation Gas Supply Section 22: HF Gas Supply Department 23: Plasma Supply Department 24: Inert Gas Supply Department 31: HF molecule 32: Free radicals generate gas 33: Free radicals

Claims

1. An etching method of selectively etching a silicon nitride film in an etching object containing the silicon nitride film and a silicon oxide film, the etching method characterized by comprising: supplying a hydrogen fluoride gas to the etching object followed by supplying radicals, thereby etching a surface oxide layer of the silicon nitride film; and, repeating a cycle of supplying the hydrogen fluoride gas to the etching object followed by supplying radicals to the surface oxide layer that has been etched, thereby selectively etching the silicon nitride film with respect to the silicon oxide film, a ratio of an etching amount of the silicon nitride film with respect to an etching amount of the silicon oxide film being a selectivity ratio, a first selectivity ratio possessed by a processing condition at the time of etching the surface oxide layer being smaller than a second selectivity ratio possessed by a processing condition at the time of selectively etching the silicon nitride film.

2. The etching method according to claim 1, characterized in that, the etching of the surface oxide layer comprises repeating a first cycle a plurality of times, the first cycle comprising: supplying the hydrogen fluoride gas to the etching object; and supplying the radicals to the etching object to which the hydrogen fluoride gas has been supplied.

3. The etching method according to claim 1 or claim 2, characterized in that, at least one of the radicals used for the etching of the surface oxide layer and the radicals used for the selective etching of the silicon nitride film is an oxygen atom-containing radical.

4. The etching method according to claim 1 or claim 2, characterized in that, the cycle of supplying the hydrogen fluoride gas to the etching object followed by supplying the radicals to the surface oxide layer that has been etched is a second cycle, a supply amount of hydrogen fluoride molecules supplied to the etching object at a time at the time of etching the surface oxide layer is a first supply amount, a supply amount of the hydrogen fluoride molecules supplied to the etching object at a time in one of the second cycles at the time of selectively etching the silicon nitride film is a second supply amount, the first supply amount is larger than the second supply amount.

5. The etching method according to claim 1 or claim 2, characterized in that, before etching the surface oxide layer, further comprising heating the etching object to a temperature of 80°C or higher and 400°C or lower, at the time of etching the surface oxide layer and selectively etching the silicon nitride film, the temperature of the etching object is maintained in a range of 80°C or higher and 400°C or lower.

6. The etching method according to claim 1 or claim 2, characterized in that, the cycle of supplying the hydrogen fluoride gas to the etching object followed by supplying the radicals to the surface oxide layer that has been etched is a second cycle, the selectively etching of the silicon nitride film comprises: in one of the second cycles, etching the silicon nitride film by a thickness of more than 0 nm and less than 5 nm.

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